Automatic track following (ATF) provides precise tracking for magnetic tape read/write operations to insure accuracy of information transfer. Standard format tapes contain ATF marks for detection by ATF systems.
A standard format for digital data storage (DDS) using 3.81 mm digital audio tape (DAT) magnetic tape is set forth by the European Computer Manufacturers Association in the document "Flexible Magnetic Media for Digital Data Interchange" (ISO/IEC JTC 1/SC 11 N 1026, hereinafter "DDS standard", Jul. 13, 1990).
Briefly, DDS format data has two types of separator marks indicating logical separations of the data. Separator 1 is a "file mark" and separator 2 is a "set mark". User data, separator marks, and associated information are formed into groups occupying groups of tracks in a "main zone" of the track. Additional information about the contents of the group, the location of the tracks and the contents of the tracks is recorded in two parts of each track called "sub zones". The two sub zones constitute the "sub data" area of the track. In addition, there are margin zones at the extreme ends of the tape and Automatic Track Finding (ATF) zones between the sub zones and the main zone. Each zone in a track is further segmented into blocks called margin blocks (in the margin zone), preamble, sub data, and postamble blocks (in the sub zones), spacer and ATF blocks (in the ATF zone), and preamble and main data blocks (in the main zone). A "frame" is a pair of adjacent tracks with azimuths of opposite polarity (where the azimuth is the angle between the mean flux transition line with a line normal to the centerline of the track). Data to be recorded is grouped into "basic groups" of 126632 bytes stored in 22 frames. Each basic group is identified by a running number from 1 to 65535. Data and separator marks are grouped into the basic groups starting with basic group no. 1. Error Correction Codes (ECC), termed C1 and C2, are computed bytes added into the data fields. ECC C3 is one extra frame added to the 22 frame groups and is capable of correcting any two tracks in a group which are bad.
Coding on magnetic tape is performed by a formatter device which codes and writes bits represented by magnetic flux reversals on a ferromagnetic tape medium. There are many different types of coding used, varying according to polarities (return to zero or not), bit train compression, and clocking capability. The most common coding schemes for high-performance tapes are non-return-to-zero-inverted (NRZI), phase encoding (PE), and group coded recording (GCR), which is a combination of NRZI and PE. A code is self-clocking if a signal pulse is generated for every stored bit.
Write data channel functions, including coding and error correction code, are typically performed by a controller operating through a write amplifier positioned near the write head. The write amplifier drives the write current through the write head.
Read data channel functions, including amplification and equalization of the read signals and data retrieval, are typically performed by automatic track-oriented gain-adjustment by a read amplifier and timing, deskewing, decoding, error detection and correction by a controller. The fundamental function of readback is to accurately convert the amplified read signal waveform into its binary equivalent. During writing, an external clock (oscillator) spaces recorded bits. An accurate readback therefore must be synchronous, and a code which inherently strobes the readback signal is desirable, such as self-clocking pulse generation in PE and GCR. One form of coding used in digital data audio tape storage is so-called 8-10 conversion GCR.
The purpose of automatic track finding (ATF) is to provide tape positioning accuracy for read/write operations. ATF blocks are allocated to two zones of a track: the ATF zone 1 and the ATF zone 2 preceding and following the main zone, respectively. ATF blocks are preceded and followed by three spacer blocks and consist of 360 channel bits. Each ATF zone consists of a combination of four signals having different channel bit patterns recorded at different physical recording densities. These signals are: (1) ATF pilot signal f1 which has a repeat pattern of 1 followed by 35 zeros; (2) AFT sync signal f2 (100000000, recorded only on track 1); (3) ATF sync signal f3 (100000, recorded only on track 2); and (4) ATF space signal f4 (100). An example of a tracking error detection procedure is: First, the frequency and length of the ATF sync signal is detected. Next crosstalk from the ATF pilot signal of an adjacent track is sampled. A fixed period later, the crosstalk signal from the ATF pilot signal of the other adjacent track is sampled. The tracking error is the difference between the levels of those two crosstalk signals. (Note that track 1=track A=+azimuth track and track 2=track B=-azimuth track.)
Conventional ATF circuits utilize an analog band pass filter to detect the tracking sync mark (f2 on A tracks, f3 on B tracks). The sync marks are detected within the ATF field. Once the tracking sync mark is detected, a series of timing signals are generated for sample-and-hold circuits. Separate sample-and-hold stages are used to capture the levels of the pilot signal (f1) crosstalk from the two adjacent tracks. An op amp and a final sample-and-hold stage capture the difference between the two pilot signals, generating the tracking servo error signal for a servo loop implemented in analog circuits. In the case of non-ideal track-to-track positioning, timing strobes from the detection of the sync signal may result in incorrect samples. In fact, under the worst case track-to-track offsets, the pilot signal of the prior track will be missed completely and the pilot signal of the following track will be sampled twice, thereby injecting erroneous information into the tracking servo loop. Also, the sync signal may arrive before the prior track pilot sample is closed, and a sync signal shock wave from the sync signal through the pilot filter will produce an erroneous offset in the ATF servo loop.